Oscillation of a closed basin on (a) a non-rotating and (b) a rotating earth (northern hemisphere).

On a non-rotating earth the tidal wave is a standing wave with a node, ie a line of no vertical movement, in the centre of the basin. Water moves back and forth horizontally at the node and vertically at both ends (This is indicated by the arrows). Co-oscillation tides (see under next heading in the text) in small bays which are too small for the earth´s rotation to come into play, can be of this type.

On a rotating earth the node is reduced to a point, called the amphidromic point. The tidal wave rotates around the amphidromic point, usually clockwise in the southern hemisphere and anti-clockwise in the northern hemisphere. There is only one point without vertical movement, the amphidromic point.

You can see the different behaviour of standing waves on a rotating and non-rotating earth in an animation.

© 1996 M. Tomczak

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